The present invention relates to methods for making semiconductor devices, in particular, semiconductor devices that include high-k gate dielectric layers.
MOS field-effect transistors with very thin silicon dioxide based gate dielectrics may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high dielectric constant (k) dielectric materials, instead of silicon dioxide, can reduce gate leakage. A high dielectric constant is greater than 10. Such a dielectric may not, however, be compatible with polysilicon—the preferred material for making the device's gate electrode.
When the gate dielectric includes a high-k film, a thin layer of silicon dioxide or silicon oxynitride may be formed between the channel and the high-k film to maintain acceptable electron mobility on the high-k film. When an electrically very thin gate dielectric is comprised of such a buffer layer, the buffer layer must be extremely thin, e.g., less than about 10 Angstroms thick. When such an ultra-thin high-k film comprises an oxide, it may manifest oxygen vacancies and excess impurity levels. Oxygen vacancies may permit undesirable interaction between the high-k film and the gate electrode. When the gate electrode comprises polysilicon, such interaction may alter the electrode's workfunction or cause the device to short through the dielectric.
Accordingly, there is a need for an improved process for making a semiconductor device that includes a high-k dielectric.